1. Field of the Invention
The present invention relates to an output control device for an electric power source, having a converter circuit for converting an input voltage from the electric power source to an output voltage and, in particular, to an output control device for protecting an electric power source by preventing malfunction due to a sharp drop in the input voltage from the electric power source having a relatively large internal impedance.
2. Description of the Related Art
Output control device control an input from an electric power source such as a fuel cell with a switching power supply circuit thereof, thereby controlling an output to a load. Known output control device typically include an output short-circuit protective circuit, an output excessive current protective circuit, or an input excessive voltage protective circuit, etc.
These protective circuits protect the switching power supply circuit, or a circuit, electrical components, and devices connected thereto as a load, but does not protect the electric power source.
The output short protective circuit and the output excessive current protective circuit limit input power from the electric power source, namely, these protective circuits limit the input power to the switching supply circuit. The level of protection provided by the protective circuit for protecting the electric power source is not satisfactory because the precision of the circuit for limiting power is poor, and because the protective circuit does not monitor directly an input voltage Vin and an input current Iin.
Japanese Patent Application Publications Nos. 10-284102 and 11-144749 disclose techniques for protecting an electric power source such as a fuel cell.
According to Japanese Patent Application Publication No. 10-284102, a function generator is arranged to input an output command to an inverter to set an upper limit value of a direct current responsive to the output command. A direct current detector is arranged to detect a direct current supplied from a fuel cell to the inverter. The inverter is controlled so that the direct current value detected by the director current detector does not exceed the upper limit value of the direct current from the function generator.
According to Japanese Patent Application Publication No. 11-144749, a second voltage value, higher than a first voltage value set for stopping an abnormal operation of a fuel cell, is set to monitor the voltage of the fuel cell. If the voltage of the fuel cell reaches the second voltage value, an input current to a power adjuster is gradually lowered.
In accordance with the disclosed techniques, if the input current to the inverter or the power adjuster is lowered, the input voltage thereto rises. As a result, the output current falls resulting in the output power being insufficient to drive a load.
Generally speaking, if an output current or an output power increases in an electric power source, such as a fuel cell having a relatively high internal impedance, the output voltage of the electric power source falls due to a voltage drop across the internal impedance.
An output control device is typically used to control the output of the electric power source. The electric power source switches the output voltage of a electric power source such as a fuel cell before providing the output to a load. As an output current Iout output from the output control device to the load increases, an output current of the electric power source, namely, an input current Iin to the output control device can rise above a predetermined value. If the input current Iin rises above the predetermined value, an output voltage of the electric power source, namely, an input voltage Vin to the output control device is lowered, leading to a positive feedback operation in which the input current Iin to the output control device further increases while the input voltage Vin further decreases. As a result, an input voltage Vin continuously decreases, and the output control device is unable to maintain the output voltage Vout at a desired value.
More specifically, when the output current Iout is low, the input current Iin is also low, and therefore the input voltage Vin is high. Under this condition, variations in the input current Iin and the input voltage Vin due to a variation in the output current Iout are small.
However, a high output current lout causes a high input current Iin, leading to a low input voltage Vin. Under this condition, variations in the input current Iin and the input voltage Vin due to a variation in the output current Iout becomes large.
In other words, variations in the input current Iin and the input voltage Vin gradually become large as the output current Iout rises. If the magnitude of the variations rises above a value unique to a switching power supply, a positive feedback chain of a rise in the input current Iin→a fall in the input voltage Vin→a rise in the input current Iin takes place. As a result, the input voltage Vin sharply drops, damaging the electric power source and leading to the output of insufficient power to a connected load.
Such a malfunction occurs in all power supply having an internal impedance. The higher the internal impedance of the electric power source, the more pronounced the malfunction becomes.